1. Field of the Invention
The present invention relates to a new and improved method for sealing boreholes in the earth and more specifically, for sealing around the outside of an elongated well casing extending into a larger diameter borehole in the earth. The present invention is concerned with sealing off dry or dormant wells such as water and oil wells and in addition, is used for sealing off casings extended into a borehole against the entry of moisture or other unwanted liquids so that instrumentation may be contained within the well casing and can be maintained in a liquid-free environment. The invention is particularly useful with ground water monitoring wells and is also concerned with sealing around an elongated well casing extending into a borehole at particular selected levels in the borehole to provide one or more zones in the borehole that are free from the entry of unwanted liquid seeping into the borehole from the earth's strata immediately adjacent thereto. The invention is also concerned with conserving the life and longevity of well casings in wells which may be kept dormant for long periods of time in order to prevent corrosion and deterioration of the well casing so that when the wells are uncapped, normal operation and production can proceed without delay. Moreover, the invention is also useful in establishing seals and plugs in underground piping.
2. Description of the Prior Art
In the past, many well casings extending into oil well boreholes have been capped off and the wells left dormant in view of the decline in oil prices. Water wells have often been capped off because of the entry of unwanted salt water or other liquids into the well casing or borehole from above or from various levels in the borehole. In particular, hollow well casings may be used for monitoring the level of the water table in the surrounding earth. This type of application requires a liquid-free environment in the well casing at least at certain levels or level ranges in the borehole so that instrumentation can be utilized for monitoring the precise level of the water table in the earth. Sealing around well casings in boreholes has previously been attempted by introducing a liquid-swellable material, such as granular bentonite in small pellet form, around the well casing. When the pellets encounter liquids, they swell up in size, however, because the pellets are free-flowing when in a dry condition, it is difficult to control the loss of pellets which tend to flow freely out into voids in the strata around the borehole. This loss of material is costly and in many instances, sealing around the well casing is unsatisfactory because the level of the pellets in a borehole is difficult to control and, thus, establishing a sealed zone around a well casing in a the borehole at a particular strata level is very difficult to obtain with any precision when free-flowing pelletized material is used.
U.S. Pat. No. 4,669,536, issued June 2, 1987, discloses the use of an elongated, cylindrically shaped, bentonite slug which is placed on a PVC casing within a borehold to prevent ground water contamination in a ground water monitoring system. Because of the elongated shape of the slug, difficulty may be encountered in moving the slug downwardly to the proper depth on a PVC casing, particularly if the casing is bent out of line along its length. Moreover, because of the elongated shape of the slug, difficulty may be encountered in fully sealing off borehole wall sections which are irregular in shape or which have large crevices. Because of the elongated body, the slug may require a considerable time period in order to become fully wetted by fluids in the borehole, so that adequate expansion to effect a seal will take too long or be incomplete. Additionally, because of the elongated shape of the slug, when liquid is encountered, the slug may expand inwardly with such force at an intermediate level thereof so as to constrict or collapse the PVC casing it surrounds.